1. Field of the Invention
The present invention relates to Treponema pallidum fused antigen and an assay for anti-Treponema pallidum antibodies, using the same fused antigen.
2. Discussion of Background
Syphilis is an infectious disease caused by Treponema pallidum. Treponema pallidum belongs to the order Spirochaetales. It has been established that several kinds of surface antigens exist on the cell surface as antigens of Treponema pallidum (The Journal of Immunology, Vol. 129, p. 833-838, 1982; The Journal of Immunology, Vol. 129, p. 1287-1291, 1982; Journal of Clinical Microbiology, Vol. 21, p. 82-87, 1985; Journal of Clinical Microbiology, Vol. 30, p. 115-122, 1992).
When the body is infected with Treponema pallidum (hereinafter referred to as Tp), anti-Tp antibodies are evoked and produced in the blood by those surface antigens, so that the diagnosis of syphilis is made by inspecting the presence or absence of the anti-Tp antibodies in the blood.
Generally immunoassays using the antigen-antibody reaction between the Tp antigens and the anti-Tp antibodies in the blood are utilized in order to detect the presence or absence of the anti-Tp antibodies in the blood of the patient.
For conducting the immunoassay for the Tp antibodies, a large amount of the Tp antigens is required. However, a large amount of Tp cannot be cultivated in vitro, so that conventionally Tp is inoculated into the testes of the rabbit and the Tp antigen is obtained from the testes of the rabbit infected with Tp and purified for use (Acta Pathol Microbiol Scand [B], Vol. 83, p. 157-160, 1975).
The above-mentioned rabbit testicular inoculation method, however, has the problems that the Tp antigen is contaminated with impurities since Tp is cultivated in vivo in the rabbits, and the cultivated Tp varies due to individual differences of rabbits, and it is extremely difficult to obtain a large amount of Tp antigen with excellent reproducibility.
Recently, because of the development of genetic engineering, the technology of artificially mass producing the Tp antigens has been developed by cloning a gene which encodes the surface antigen of Tp (Science, Vol. 216, p. 522-523, 1982; Infection and Immunity, Vol. 36, p. 1238-1241, 1982; Infection and Immunity, Vol. 41, p. 709-721, 1983; Infection and Immunity, Vol. 42, p. 435-445, 1983; Infection and Immunity, Vol. 42, p. 187-196, 1983; Journal of Bacteriology, Vol. 162, p. 1227-1237, 1985; Infection and Immunity, Vol. 54, p. 500-506, 1986; Infection and Immunity, Vol. 56, p. 71-78, 1988; Infection and Immunity, Vol. 57, p. 2612-2623, 1989; Infection and Immunity, Vol. 57, p. 3708-3714, 1989; Molecular Microbiology, Vol. 4, p. 1371-1379, 1990; Infection and Immunity, Vol. 58, p. 1697-1704, 1990; Infection and Immunity, Vol. 61, p. 1202-1210, 1993; Laid-Open Patent Application 2-500403).
By using the technology of genetic engineering, the Tp antigen can be mass produced without using living animals. However, with respect to some kinds of Tp surface antigens, almost no product is expressed by using only the gene which encodes the same antigen itself.
Therefore, in the above-mentioned case, a method of expression of the desired antigen has been proposed using a fused gene which is prepared by joining a gene such as thioredoxin derived from Escherichia coli (hereinafter referred to as TRX) or a glutathione S-transferase derived from Schisstosoma japonicum (hereinafter referred to as GST) and a gene of the desired material as disclosed in Japanese Laid-Open Patent Application 5-507209 and Japanese Laid-Open Patent Application 1-503441.
It has been discovered that a GST15 kDa antigen which is a fused gene of GST and a 15 kDa antigen which is one surface antigen of Tp, and a GST17 kDa antigen which is a fused gene of GST and 17 kDa antigen which is another surface antigen of Tp, exhibit high sensitivity when used for the assay of the anti-Tp antibody as disclosed in Japanese Laid-Open Patent Application 7-63365.
Since Escherichia coli and Schisstosoma japonicum can live in the human body and therefore there are many people who have a factor in the blood which reacts with TRX or GST. In such a case, even if the person is not infected with Tp, a positive reaction is exhibited with respect to infection with Tp. Thus, it is evident that this has a serious effect on the diagnosis of infection with Tp.
At present, syphilis is completely curable because of the development of antibiotics. Therefore, it is desired that syphilis be cured based on a quick and accurate diagnosis of syphilis. In order to achieve this, there is a keen demand for an assay of Tp antigens and anti-Tp antibodies, with high sensitivity and specificity.